1. FIELD OF THE INVENTION
This invention relates to fuel elements for nuclear reactors and, more particularly, to methods and apparatus for inserting and withdrawing fuel rods from fuel element grid structures, and the like.
2. SUMMARY OF THE PRIOR ART
In order to function, nuclear reactors must have an inventory of fissionable material that will sustain a continuous sequence of fission reactions. Frequently, the uranium, or other nuclear fuel, is loaded into long, hollow and slender metal rods that are termed "fuel rods". These loaded fuel rods then are mounted together into groups each of perhaps 200 rods to form "fuel elements". A number of these fuel elements, when assembled, form one array which comprises the reactor core that provides the concentration of fissionable material which is needed to continue the fission process.
The fuel rods within the fuel elements usually are subjected to a number of adverse environmental conditions during reactor operation. In this respect, the heat generated in the fuel rods often is removed by means of the primary cooling water that flows through the reactor core in a direction which is parallel to the longitudinal axes of the fuel rods. Especially in connection with power reactors, the water flow velocity and the flow rate must be very high in order to remove the large quantity of heat that is generated. The surface area of the individual fuel rods, moreover, must be as fully exposed as possible to the flowing water in order to promote a high thermal conductivity between the fuel rod and the primary coolant and to prevent the development of "hot-spots" on the fuel rod due to poor local flow conditions, or the like.
Thus, fuel element structures are confronted with the need to satisfy two essentially conflicting requirements; the need to stabilize a large number of long, dense, nuclear fuel-filled thinwalled tubes that are exposed to the vibratory and other forces which are caused by very high cooling water flow rates, and the need to reduce the structural restraints on these fuel rods to a minimum in order to promote heat transfer from the rods to the coolant. To satisfy these essentially conflicting needs, fuel element grids often are used to stabilize the array of fuel rods within the grid structure. Usually, these grids comprise a cellular structure that is formed through the mutually perpendicular intersections of a group of interlocking metal plates. One fuel rod is lodged in each of the cells thus formed in the grid structure. Bosses and the like protrude from the surfaces of the portions of these interlocking plates that form the individual cell walls. These bosses engage the outer surface of the fuel rod within the particular cell and serve to restrain rod motion.
These bosses are of two basic types. One type of boss is of a very resilient character, being essentially spring-mounted. These resilient bosses permit the fuel rods to be inserted into the grid structure with relative ease. During reactor operation, however, the resilient nature of the boss mountings enables the bosses to move relative to the adjacent fuel rod surfaces. This motion produces an undesirable wearing or "fretting" of the rod surface that weakens the rod structure and can cause a failure.
The other type of boss is a very stiff and non-resilient arrangement that essentially eliminates relative movement between the fuel rods and the respective bosses. These stiff bosses, although eliminating "fretting" problems, nevertheless, lead to other difficulties. In this respect, it is difficult to lodge a fuel rod within a grid cell without scraping and gouging the rod surface against the relatively unyielding bosses. Scratches of this nature also weaken the fuel rod structure and establish corrosion loci, too.
Thus, there exists a need to provide an efficient and economical means for inserting fuel rods into the cells of a fuel element grid that has protruding non-resilient bosses without marring the rod surfaces.